Storage containers, such as pouches or bags, in particular for medical purposes such as intravenous solutions, blood and plasma bags and the like are used in the millions and are typically made from thermoplastic film materials sealed at their sides or ends. The thermoplastic film materials are typically sealed together using a variety of heat sources including, for example, impulse heaters and electrical resistance heaters. Impulse heaters and electrical resistance heaters are used for sealing a wide range of thermoplastic materials. Unfortunately, impulse heaters and electrical resistance heaters are unable to dissipate heat very rapidly and, therefore, there is no real cooling cycle and the thermoplastic material cannot be held under cold pressure until the seal sets.
Another type of sealing is achieved through dielectric welding or sealing techniques. Such techniques may include the dielectric heating of multiple layers of thermoplastic film in order to melt and seal adjacent layers together. The electric current may be supplied by a pair of electrodes positioned in opposing relationship above and below the films to be sealed. Typically, these electrodes are powered by a radio frequency electrical signal. Radio frequency is abbreviated herein as RF.
Generally, the conventional method for forming seals in thermoplastic films includes a sealing cycle having multiple phases. First, there is a positioning phase wherein at least two layers of thermoplastic material or film are positioned in superposed relation between the top and bottom electrodes. Typically, the top electrode is a metallic electrode formed in the shape of the desired sealing pattern and the bottom electrode formed as a metal platen of a pneumatically operated press.
During the contacting phase, the electrodes are brought together in contact with the films.
Upon contact, RF energy is applied to the electrodes and causes heating and softening or melting of the films in the region defined by the shape of the top electrode. The flow of RF energy constitutes the heating phase, and in a conventional apparatus, involves only dielectric loss heating generated within the films until a seal is formed. The metallic electrode and metal platen do not become hot from the influence of the high frequency energy and instead draw heat from the material being sealed.
Once sufficient melting or softening of the films has occured, the RF power to the electrodes is interrupted to allow for cooling and resolidifying of the films in or near the region defined by the top electrode. At this phase, each electrode acts as a thermal heat sink for the heated films and resolidifying under pressure provides a good seal.
Finally, during the release phase, the electrodes are separated and the films, now sealed, are removed.
Sealing thermoplastic film materials with electromagnetic energy at radio frequencies has the advantage of rapid cooling, but RF use has been limited to sealing of those materials having a certain dielectric dissipation factor as further discussed below.
The dielectric dissipation factor for a plastic film determines how much of the RF energy is converted into heat within the film. The dielectric dissipation factor is a function of the material's dielectric constant and the loss tangent (tan .delta., where .delta.=90.degree.-.theta., and where .theta. is the phase angle) defined as .sigma./.omega..epsilon..sub.f, where .sigma. is the film's electrical conductivity, .omega. is the frequency of operation, and .epsilon..sub.f is the film's electrical permittivity. Electrical permittivity is another name for dielectric constant.
The use of films having relatively high dielectric dissipation factors (0.04-0.10), referred to herein as "high loss films", is generally preferred in conventional dielectric sealing because these films form a dielectrically lossy load and become hot and melt when subjected to RF energy. Examples of high loss films include, for example, films made from polyurethane polymer resin, vinylidene chloride copolymer resin, and polyvinyl chloride polymer resin (PVC). In contrast, films having a dielectric dissipation factor below 0.04, herein referred to as "low loss films", generally have poor heat generating characteristics and cannot be readily or satisfactorily sealed when subjected to heating by RF energy alone.
It is recognized that high loss films such as films made from polyurethane polymers are desirable materials for medical applications, but are often not used in medical applications due to their high cost. Other high loss films such as those made of PVC type polymers, for example, are also used for medical applications but it is desired to do away with them because of concerns that they contain chlorine ions which pose an environmental risk when disposed of. Another concern is that PVC films suffer from plasticizer migration over time, and thus the plasticizer could leach into a solution contained in a PVC container thereby contaminating the solution. These solutions are typically intended for injection into a human; so if contaminated, the solution cannot be used.
Low loss films, such as those of polyethylene, polypropylene, and linear low-density polyethylene, on the other hand, do not have these adverse characteristics, but are generally poor candidates for conventional dielectric sealing because of their low loss characteristics. It is therefore desirable for a sealing apparatus to be capable of sealing low loss films at high speed and with the seal quality attendant high loss films.
However, known apparatuses for dielectric sealing have not been able to seal low loss films because heat generated in such film when subjected to an RF field is insufficient to cause the film to soften or melt. It has been found that this limitation of a conventional apparatus is dependent upon the electrode utilized in the sealing apparatus as well as the inefficient design of the apparatus.
A major source of energy inefficiency in a sealing apparatus for low loss films is the source-to-load impedance mismatch which arises between the RF signal generator and the sealing electrodes. For example, when an RF signal generator is imperfectly coupled to the sealing electrodes via a transmission line a portion of the load-directed power ("P.sub.ld ") can be reflected back towards the signal generator or become source-directed ("P.sub.sd "). It is known that imperfect coupling can substantially increase the power dissipated in the signal generator and thereby reduce its life span and efficiency.
The maximum efficiency of power transfer to the sealing electrodes will occur when the transmission coefficient .tau. is maximized. The transmission coefficient can be expressed according to the following relationship: EQU .tau.=2Z.sub.12 /(Z.sub.12 +Z.sub.0) (1)
Where, Z.sub.12 is the input impedance of the sealing electrodes and Z.sub.0 is the impedance of the transmission line.
The speed by which films can be sealed is directly related to the magnitude of the voltage ("V.sub.e ") developed across the electrodes and intervening plastic films during the sealing cycle. This voltage should be as high as possible without causing breakdown of the films. Moreover, the magnitude of V.sub.e is a function not only of .tau., but also of the energy stored in passive components, such as any inductors or capacitors connected to the electrodes. It is therefore preferable that a sealing apparatus be capable of operating at or near the maximum V.sub.e possible.
Conventional apparatus for sealing high loss thermoplastic films include, for example, sealing electrodes connected in parallel with a compensation inductor, or in series with a compensation inductor as described in U.S. Pat. No. 4,629,851 to Holle. These parallel or series connected components may cause V.sub.e to reach a maximum value at a specified point in the sealing cycle; however, because the effective capacitance of the electrodes changes during the sealing cycle, optimum sealing efficiency is not obtained using these conventional configurations since V.sub.e cannot be maintained at its maximum value throughout the entire cycle.
It is an object of the invention to provide an apparatus and method for radio frequency sealing thermoplastic films together.
It is another object of the invention to provide a radio frequency sealing apparatus and method capable of sealing thermoplastic material having a dielectric dissipation factor near or below 0.04.
It is a further object of the invention to provide a radio frequency sealing apparatus and method being impedance matched to the radio frequency generator and operable with low reflected power.
It is a still further object of the invention to provide an improved electrode to be utilized in a sealing apparatus and method for radio frequency sealing two or more films of thermoplastic material together in arbitrary shapes.
Therefore, according to one aspect of the invention, there is provided an apparatus for sealing two or more films of thermoplastic material together comprising: a first and a second electrode adapted for receiving and contacting said films therebetween; generating means electrically connected to said first and second electrodes for supplying radio frequency power to said electrodes; and matching means connected between said generating means and said electrodes for impedance matching said generating means to said electrodes.
According to another aspect of the invention, there is provided an apparatus for sealing two or more films of thermoplastic material together comprising: a first and a second electrode adapted for receiving and contacting said films therebetween; generating means electrically connected to said first and second electrodes for supplying radio frequency power to said electrodes and including means for measuring said radio frequency power delivered to said electrodes.
According to yet another aspect of the invention, there is provided an electrode for use in an apparatus for radio frequency sealing thermoplastic films together, said electrode having a coating of ceramic material.
According to a further aspect of the invention, there is provided a method of sealing two or more films of thermoplastic material together comprising steps of: positioning said films in superposed relation between a first and a second electrode; pressing said films together with said first and second electrodes, where at least one of said electrodes has a coating of ceramic material contacting said films; supplying radio frequency power to said electrodes thereby generating dielectric heating and conductive heating of said films; and removing said radio frequency power while maintaining pressure of said electrodes on said films until said thermoplastic material has cooled sufficiently to set whereby a seal is formed.
According to a still further aspect of the invention, there is provided a method of sealing two or more films of thermoplastic material together comprising steps of: positioning said films in superposed relation between a first and a second electrode; pressing said films together with said first and second electrodes; supplying radio frequency power to said electrodes thereby generating dielectric heating of said films; measuring said radio frequency power delivered to said electrodes and controlling said radio frequency power to produce a readily reproducible seal.
In view of the undesirable characteristics of certain thermoplastic materials such as polyvinyl chloride, which use plasticizers in the films, for making medical supply bags and the undesirable heating cycle of impulse heaters and resistance heaters when sealing, it is desirable to use other thermoplastic materials and sealing methods to make medical supply bags.
The radio frequency sealing method and apparatus of this invention are especially useful in the construction of medical containers, such as collection bags, intravenous solution bags, blood and plasma bags, and the like. According to the present invention, thermoplastic materials with a low dielectric dissipation factor may be sealed together with RF energy using the RF electrode of this invention. One example of a preferred thermoplastic material having a low dielectric dissipation factor for use in this invention is polyethylene.
In one particular embodiment, the apparatus of the present invention comprises a ceramic coated top electrode having the shape of the seal to be formed and a bottom electrode for sealing low loss thermoplastic films. The ceramic coating acts as an intermediate dielectric layer between the films and top electrode, when compressed. When exposed to RF energy, the ceramic coating becomes hot and provides thermal conduction to the low loss films. In addition, the heating by thermal conduction also increases the loss properties of the films. Although it is not intended to be bound to any theory, it is believed that the thermal conduction from the hot ceramic coating acts synergistically with the heat generated from within the low loss films and is sufficient to melt the films and form a seal. In embodiments wherein only one of the electrodes includes a ceramic coating, a thermal insulating layer can be provided on the uncoated electrode to prevent that electrode from acting as a heat sink to the films during the heating phase.
The improved electrode of this invention may be made from a metal selected from the group consisting of the transition metals, Group III A metals, or combinations thereof, such as tungsten carbide, aluminum, copper, or brass. The electrode is coated with a ceramic oxide selected from the group consisting of transition metal oxides, Group III A metal oxides, or combinations thereof, and preferably is aluminum oxide. Then, the transition metal oxide or Group III A metal oxide is coated with a release material, and preferably the release material is selected from the group consisting of silicone, polytetrafluoroethylene or combinations thereof.
The apparatus of the present invention preferably includes a compensation circuit connected in parallel with the electrodes for providing a negative reactance and maintaining the overall reactance seen by the electrodes at a near constant value throughout the sealing cycle even though the capacitance of the electrodes ("C.sub.e ") increases as the films are compressed.
A matching circuit is also interposed with the electrodes. The elements of the matching network may be interconnected with the electrodes in a variety of configurations, such as parallel or series-parallel, to achieve the desired results. This circuit provides a sufficient positive reactance to cause resonance and matches the impedance to the characteristic impedance of the signal line and the signal generator. Typically, the signal line is a coaxial transmission line having a characteristic impedance in the range of 50-300 ohms. The range of 50-300 ohms is characteristic of commercial coaxial transmission lines. Thus, it is not intended to be limited thereby, as other ranges of impedance may be typical of other lines.
The method for making seals between thermoplastic films according to the invention includes positioning at least two thermoplastic films in superposed relation between a first electrode and a second electrode with at least one of the electrodes having a ceramic coating thereon. A radio frequency signal is applied to the electrodes to thereby both dielectrically heat the thermoplastic films and conductively heat same by heat generated within the ceramic coating. The power delivered by the electrodes may be advantageously measured and controlled thereby to produce a readily reproducible seal.
Because of the impedance matching, it is now possible to measure the power. Measuring allows for monitoring, and monitoring is desirable in order to obtain accurate statistical data about the quality of the seals formed in low loss material. The RF monitoring of the forward and reflected power may be continuously measured using a bi-directional coupler, for example. Thus, with the instant invention the sealing cycle efficiency can be controlled and electrical arcing through the films to be sealed can be prevented. Accurate control of sealing cycle efficiency cannot be obtained with conventional RF sealing devices.